As she explains, the idea is to keep distinct activities contained within distinct desktops. To get a better idea of how it works in practice, check out those five desktops in use (with notes). You may not need distinct wallpaper to separate activities into different desktops (see the context-configured desktop), but as a devout virtual desktop user, I love the idea. If you want to build your own version, check out the tutorial she followed and create some backgrounds yourself.